As an oil well or the like is being drilled, drilling mud is pumped down the drill string to the rock bit that is drilling the well. The drilling fluid such as high density mud exists the drill string at the rock bit and flows to the ground surface in the annulus between the drill string and the bore hole or casing within the bore hole. At the ground surface the drilling mud from the annulus is passed through a high pressure choke or valve, through which a large pressure drop occurs. This maintains a high pressure in the well bore for mud flow control and control of the well.
It is often important to maintain a selected pressure of the drilling mud at the ground surface so that, in combination with the hydrostatic head in the well bore, a desired pressure is maintained at the drill bit. The pressure desired in a given situation depends on the drilling parameters and the rock formations being drilled. A selected minimum pressure is desired for control of the well and to assure proper drilling. A selected maximum pressure is desired for safety reasons and to prevent damage to subterranean rock formations. During drilling various conditions may cause an upward "kick" in pressure and it is desirable to relieve that pressure promptly and in a controlled manner.
For example, pressure may increase in the event a particle of the formation being drilled lodges in the choke and obstructs discharge of drilling fluid. In such a situation it is desirable to open the choke until the particle is dislodged and then close the choke to bring pressure back into the desired range.
It is customary to measure the pressure of the drilling mud being pumped into the drill string and use that pressure as indicative of the desired bottom hole pressure. Control of pressure is obtained by changing the opening of the choke. There is a time lag between a change in choke position and the pressure at the inlet to the drill string. This time lag is in the order of one second per thousand feet of hydraulic line. Thus, for example, if a well is 5000 feet deep, it takes about ten seconds for each change in choke position to be reflected in the pressure at the mud inlet.
Any system for controlling pressure in such a situation needs to gradually change choke position so that overcontrol is minimized and "hunting" of the pressure for the desired range is avoided.
The chokes used on wells are often hydraulically controlled. The choke may, for example, be a needle valve with a rotatable stem. A hydraulic actuator is connected to the stem for rotating it. Hydraulic fluid flow can be used for either opening or closing the valve as required.
It is required in many drilling situations, particularly on offshore platforms and the like, to employ systems that are explosion proof; that is, systems that do not have ignition sources that may trigger a fire or explosion at the drilling site. Electrical systems used in such situations must be provided with explosion-proof housings at considerable expense. For example, an explosion-proof housing for a typical system mounted in a conventional electronics rack may add $3000 to the cost of the system.
It can therefore be highly desirable to provide an all pneumatic system that does not require special provisions to make it explosion proof. Such systems should be reliable, versatile, economical, adjustable, safe, and simple.